A conventional vortex flow meter generally comprises a fluid channel, a vortex shedder, and a vortex shedding frequency measuring device. The vortex shedder generally comprises a blunt body, such as a disk, a ring, and a T-shape. The measurement of shedding frequency is achieved by detecting the frequency of pressure fluctuation caused by periodic shedding of vortex induced downstream of the vortex shedder. The frequency measuring device is classified as invasive and non-invasive (wall-surface type). The wall-surface type frequency measuring device is becoming more popular and gradually replacing the invasive-type frequency measuring device due to the reduced pressure drop and less interference with the flow field while enhancing convenience for maintenance of the measuring device.
In the method that the conventional vortex flow meter employs to determine the flow velocity, the flow velocity is determined from a specific relationship between Strouhal number (St) and Reynolds number (Re). For example, Taiwan Patent No. 188378 discloses a linear relationship between the Strouhal number and Reynolds number, which is represented by the equation: St=0.15814+6.73×10−7×Re. However, such linear relationship only exists for the specific vortex shedder in turbulent flow with high flow rates. Thus, many researches have been devoted to design vortex shedders of different shapes and structures to improve the precision and measurement ranges of vortex type flow meters.
Some studies made on vortex induced by blunt body showed that with the increase of Reynolds number, two-dimensional vortex flow gradually undergoes a transition to a three-dimensional pattern. This causes discontinuities in the relationship curve between the dimensionless frequency and Reynolds number and is often referred to as three-dimensional vortex flow transition zone. The irregularities in the vortex shedding frequencies measured in the three-dimensional regime causes troubles and errors in measurement carried out with a vortex flow meter. Consequently, the Reynolds number threshold (ReT) of the transition zone of three-dimensional vortex flow is considered as the upper limit for the design of low flow rate vortex flow meter while the Reynolds number threshold (ReC) for the two-dimensional vortex flow is the lower limit.
Some prior references provide different designs for a vortex flow meter. For example, U.S. Pat. No. 4,453,416 discloses a blunt body having a truncated conic shape positioned in a fluid channel of a vortex flow meter. Another example is U.S. Pat. No. 6,435,036, which discloses a triangular blunt body positioned in a fluid channel of a vortex flow meter. The design features expanded measurement range, simple construction and high precision. U.S. Pat. Nos. 4,592,240, 5,170,671, and 5,351,559 all disclose a disk-shaped blunt body positioned in a fluid channel of a vortex flow meter. Further, U.S. Pat. No. 4,977,781 discloses a sharp conic blunt body positioned in a fluid channel of a vortex flow meter, which features increased measurement range, simple construction and high precision. All these prior art references are improvement of the shape and structure of the sharp-front-edged blunt body and attempt to provide enhanced fluid measurement result.
However, the design of the conventional vortex flow meters often takes no consideration of non-isothermal applications of fluid and vortex shedder and in addition, the conventional vortex flow meters are only applicable to a fixed measurement range.
Therefore, it is desired to provide a vortex flow meter that is applicable to a wide range of flow rate and overcomes the above-discussed disadvantages of the prior arts.